The heart circulates blood throughout the body in a continual cycle of electrical stimulation of cardiac muscle cells. At rest, each muscle cell accumulates an electrical charge across its cell membrane that is then depolarized during each heartbeat. Initially, the cells of the sinoatrial node in the right atrium spontaneously depolarize and create a cardiac action potential of electrical impulses that rapidly propagates outward. The cardiac action potential stimulates muscle cells of the atrial myocardium to depolarize and contract in unison in systolic contraction, after which the cardiac action potential encounters the atrioventricular node located at the juncture of the atria and ventricles near the center of the heart. The atrioventricular node slightly delays cardiac action potential propagation to ensure complete drainage of blood from the atria after which the muscle cells of the ventricular myocardium are stimulated into systolic contraction and thereby complete the heart beat cycle.
The depolarization of the muscle cells of the atrial and ventricular myocardium act as sequential voltage sources, which generate a current flow across the thoracic region of the body and result in a characteristic signal on the body surface. In a typical ECG monitor, cardiac action potentials occur between 0.05 Hz to 150 Hz with a signal strength of around 3 mVp-p (peak-to-peak). Although miniscule, the current flow can be measured to characterize the electrical activity of the heart using an electrocardiographic (ECG) monitor or similar device. Voltage differentials from pairings of the electrodes are filtered, amplified, and combined into P, QRS, and T complexes.
Conventionally, cardiac action potentials are detected through electrodes attached to the skin on the chest and limbs based on the American Heart Association's classic 12-lead placement model, such as described in P. Libby et al., “Braunwald's Heart Disease—A Textbook of Cardiovascular Medicine,” Chs. 11 and 12 (8th ed. 2008), the disclosure of which is incorporated by reference. Both traditional in-clinic and ambulatory Holter-style ECG monitors follow the standard 12-lead model with variations on numbers and placement of leads. Generally, limb lead electrodes are placed on each arm and on the left leg, while precordial lead electrodes are placed on the left upper chest region over the heart. The limb leads can be re-positioned as necessary to compensate for variability in patient anatomy due to tissue and bone density and heart position.
Accurate ECG recording requires the absence of significant ambient noise. The 12-lead model attempts to maximize cardiac action potential signal strength. However, ECG monitors are still affected by environmental noise and feedback. The body acts as an antenna that is susceptible to electromagnetic (EMF) noise, which is often caused by power lines. Cardiac action potentials are inherently weak signals easily overwhelmed by such ambient interference. Skin-to-electrode impedance is around 51 kOhms. 50 Hz or 60 Hz power line EMF interference, depending on country, is filtered from the input signal using a filter, while baseline low-frequency wander is normally corrected by using a feedback system.
Conventional monitoring circuits combine physical shielding, analog filtering, and digital filtering to reduce noise. However, noise filtering methods can cut dynamic range, particularly low frequency sensitivity, to keep signals within a permissible dynamic range. As a consequence, ECG quality and clinical value can suffer when extremely low frequency content is lost. There are a variety of analog feedback circuits in conventional ECG monitors to drive a common mode voltage and keep the amplifiers from oversaturation. For instance, in a right leg drive (RLD) circuit, a network of resistors sense common mode voltage on a body, which is then inverted, amplified, and fed back into the body through a reference electrode. Consequently, the body becomes a summing junction in a feedback loop. Negative feedback thereafter drives common mode voltage to a nominal value.
Although effective at countering respiration, wander and drift, such conventional analog RLD circuits increase circuit complexity and cost and destroy very low frequency content. Even though RLD circuits typically drive less than one microampere of current into the right leg, at a minimum, a resistor feedback network and an output op-amp that drives a reference electrode must be powered and placed in the circuit. The constant power draw to drive the circuit can tax power budget constraints, particularly where the circuit is in an ambulatory battery-powered ECG monitor.
For instance, U.S. Pat. No. 5,392,784, issued Feb. 28, 1995 to Gudaitis, discloses a virtual right leg drive circuit for common mode voltage reduction. A circuit senses common mode voltage received by inputs from a signal amplifier and generates a compensation voltage, representative of the common mode voltage. A capacitance to chassis ground receives a voltage representative of the compensation voltage. The circuit and the capacitance cause the amplifier power supply voltages to track the common mode voltage. The capacitance permits the feedback loop gain to be increased to reduce common mode voltage errors, but at the cost of increased circuit complexity.
U.S. Patent application, Publication No. 2007/0255153, filed Nov. 1, 2007, to Kumar et al., now patented as U.S. Pat. No. 8,160,682; U.S. Patent application, Publication No. 2007/0225611, filed Feb. 6, 2007, to Kumar et al., now patented as U.S. Pat. No. 8,244,335; and U.S. Patent application, Publication No. 2007/0249946, filed Feb. 6, 2007, to Kumar et al., now patented as U.S. Pat. No. 8,150,502, disclose a non-invasive cardiac monitor and methods of using continuously recorded cardiac data. A heart monitor suitable for use in primary care includes a self-contained and sealed housing. Continuously recorded cardiac monitoring is provided through a sequence of simple detect-store-offload operations. An action sequencer state machine directs the flow of information to either memory or to a switched I/O unit without feedback control. In one embodiment, a 24-bit analog-to-digital converter converts continuously detected ECG information into uncompressed 8-bit data. Amplification circuitry is not required, as amplification and scaling are replaced by selecting an 8-bit data resolution out of a possible 24-bit range. Additionally, the 24-bit to 8-bit selector serves as a scaler to keep signal excursions within the numeric range of the analog-to-digital converter and to provide image scaling to the end user. The stored ECG data can be retrieved and analyzed offline to identify ECG events. U.S. Patent application, Publication No. 2008/0284599, filed Apr. 28, 2006, to Zdeblick et al., pending, and U.S. Patent application, Publication No. 2008/0306359, filed Dec. 11, 2008, to Zdeblick et al., now patented as U.S. Pat. No. 8,547,248, disclose a pharma-informatics system for detecting the actual physical delivery of a pharmaceutical agent into a body. An integrated circuit is surrounded by pharmacologically active or inert materials to form a pill, which dissolve in the stomach through a combination of mechanical action and stomach fluids. As the pill dissolves, areas of the integrated circuit become exposed and power is supplied to the circuit, which begins to operate and transmit an information signal. A signal detection receiver can be positioned as an external device worn outside the body with one or more electrodes attached to the skin at different locations. The receiver can include the capability to provide both pharmaceutical ingestion reporting and psychological sensing in a form that can be transmitted to a remote location, such as a clinician or central monitoring agency.
Therefore, a need remains for an approach to efficiently negate the affects of environmental interference, while preserving dynamic signal range in an ECG monitor and simultaneously reducing the complexity of ECG circuitry, especially for designs intended for low-cost and disposable ECG monitoring technologies.